The crucial parameters for the use of an optical glass are the refractive index and the change in refractive index with wavelength, known as the dispersion.
The refractive index n in the central region of the visible spectrum is usually given as n.sub.d, the refractive index at the wavelength 587.56 nm, for which purpose the spectral line used is the yellow helium line.
Increasingly, however, the requirement is, as in DIN 58925, for the specified principal refractive index to be the refractive index n.sub.e at 546.07 nm, which corresponds to the green mercury line.
The change in refractive index with wavelength is given by the Abbe number: ##EQU1##
n.sub.F corresponds to the refractive index at the wavelength 486.13 nm (blue H line),
n.sub.c corresponds to the refractive index at the wavelength 656.28 nm (red H line),
n.sub.F, corresponds to the refractive index at the wavelength 479.99 nm (blue Cd line) and n.sub.c, corresponds to the refractive index at the wavelength 643.85 nm (red Cd line).
The difference n.sub.F -n.sub.C or n.sub.F' -n.sub.C' is known as the principal dispersion. Other differences represent partial dispersions. Relative partial dispersions P.sub.g,F', are the ratio between a partial dispersion and the principal dispersion. The relative partial dispersion P.sub.g,F', here based, for example, on the wavelengths g (435.83 nm, blue Hg line) and F' (479.99 nm, blue Cd line), is then given by: ##EQU2##
Like the Abbe number, the relative partial dispersion is an important material constant for an optical glass.
The majority of glasses satisfy an approximately linear relationship between P.sub.x,y and .nu., given by P.sub.x,y =a.sub.x,y +b.sub.x,y .multidot..nu. (standard straight line).
Glasses which do not satisfy this equation are known as glasses having anomalous partial dispersion. In this case, the equation must be expanded by an additional correction term .DELTA.P.sub.x,y : EQU P.sub.x,y =a.sub.x,y +b.sub.x,y .multidot..nu.+.DELTA.P.sub.x,y.
Depending on whether .DELTA.P.sub.x,y is greater than or less than 0, the glasses are known as glasses of positive or negative anomalous partial dispersion.
A suitable combination of optical glasses of different Abbe number allows imaging defects, chromatic aberration, for lens systems, for example for two colors, to be eliminated or at least improved. The residual chromatic aberration which exists for the uncorrected colors is known as the secondary spectrum. This effect is a particular disadvantage for high-performance optics, since it impairs the image sharpness and the resolution capacity of the optical system.
However, the use of glasses having anomalous partial dispersion in optical lens systems would allow the secondary spectrum to be diminished, giving corrected lens systems having excellent image sharpness and high resolution capacity.
Particularly desirable is correction in the blue region of the visible spectrum, which is characterized by the relative partial dispersion .DELTA.P.sub.g,F' already mentioned above by way of example.
All currently known glasses featuring negative anomalous partial dispersion are borate glasses.
The choice of optical glasses having high negative anomalous partial dispersion with .DELTA.P.sub.g,F' less than -0.006 is very restricted. For these glasses, a certain amassing in the dispersion region of the Abbe number around .nu..sub.d =50 is characteristic.
A broadening of the current range toward higher Abbe numbers is desired by the users, since this would enable further improved correction for optical lens systems.
The material having the greatest anomalous partial dispersion is an alum monocrystal, having a deviation of -0.04. In chemical terms, alum is potassium aluminum sulphate containing twelve molecules of water of crystallization: KA1(SO.sub.4).sub.2 .multidot.12 H.sub.2 O.
However, the hygroscopic properties of this material mean that it has not achieved any practical significance in optics; alum dissolves in its own water of crystallization at 80.degree. C. and, at low atmospheric humidity, it releases water of crystallization into the environment, even at room temperature. A further disadvantage is its low hardness.
However, adequate chemical resistance is an important criterion for any practical application of an optical glass.
For this reason, pure B.sub.2 O.sub.3 glass, the glass with the absolutely highest anomalous partial dispersion, having an extrapolated .DELTA.P.sub.g,F' value of about -0.02, cannot be employed either, since it is very difficult to prepare as a consequence of its hygroscopicity, due to its structure, and it efflorescences after only a short time in air.
An optical glass which simultaneously has high negative anomalous partial dispersion and excellent UV transmission, in particular in the short-wave UV region below 300 nm, is not known from the prior art.
Thus, DE 39 17 614 C1 discloses an optical glass having negative anomalous partial dispersion .DELTA.P.sub.g,F', a refractive index n.sub.d &gt;1.67 and an Abbe number .nu..sub.d &gt;36, having a composition (in % by weight) of SiO.sub.2 3-11, GeO.sub.2 0-3, .SIGMA. SiO.sub.2 +GeO.sub.2 4.5-11, B.sub.2 O.sub.3 29-35, Al.sub.2 O.sub.3 5-13, ZrO .sub.2 1-3, TiO.sub.2 0.2-3, Ta.sub.2 O.sub.5 0.2-1.5, PbO 30-45, Li.sub.2 O 0-3, Na.sub.2 O 0-3, K.sub.2 O 0-3, Rb.sub.2 O 0-3, Cs.sub.2 O 0-3, .SIGMA. alkali metal oxides 0-3, MgO 0-3.5, CaO 0-3.5, BaO 0-3.5, SrO 0-3.5, .SIGMA. alkaline earth metal oxides 0-6 if .SIGMA. SiO.sub.2 +GeO.sub.2 .gtoreq.9, .SIGMA. alkaline earth metal oxides 0-3.5 if .SIGMA. SiO.sub.2 +GeO.sub.2 &lt;9, ZnO 0-14, La.sub.2 O.sub.3 0-3, Nb.sub.2 O.sub.5 0-7, Sb.sub.2 O.sub.3 0-1, As .sub.2 O.sub.3 0-0.3, WO.sub.3 0-1.5 and F.sup.- 0-1.
Favorable transmission properties should only be expected here as far as the blue spectral region at the furthest; no transmission data are given for the wavelength of the i line at 365.01 nm (ultra-violet Hg line). DE 39 17 614 C1 also differs through significant parts of the glass components, such as, for example, through a considerably lower B.sub.2 O.sub.3 content.
East German Patent 1603 07 relates to an optical crown glass having a refractive index n.sub.e =1.500-1.555 and an Abbe number .nu..sub.e =57-62, negative anomalous partial dispersion .DELTA..nu..sub.e &lt;-7, and increased crystallization and chemical resistance, and it is suitable for correction of the secondary spectrum in optical systems and contains at least the components B.sub.2 O.sub.3 -CaO-Li.sub.2 O and/or Na.sub.2 O and Al.sub.2 O.sub.3, with the composition (in % by weight): B.sub.2 O.sub.3 73.0-87.0; Li.sub.2 O and/or Na.sub.2 O 3.0-5.0; Al.sub.2 O.sub.3 4.0-9.0; CaO 2.0-5.0; MgO 0-5.0; La.sub.2 O.sub.3 0-9.0; ZrO.sub.2 0-4.5.
The very high B.sub.2 O.sub.3 content of 73-87% by weight and the low and thus not structure-optimal Al.sub.2 O.sub.3 content of 4-9% by weight mean that the chemical resistance of these glasses, in particular for those having .nu..sub.e of greater than 60, is inadequate in practice. The possibility of adding SiO.sub.2 as a stabilizer, which can have a favorable effect on the chemical resistance without the anomalous partial dispersion properties being lost, is not suggested in the East German Patent.
JP 60-46946 discloses UV-transparent glasses of the borosilicate type which are predominantly in the quaternary system CaO.multidot.Al.sub.2 O.sub.3 .multidot.B.sub.2 O.sub.3 .multidot.SiO.sub.2. The glasses presented here have, apart from Example Nos. 10 and 11, excessively high SiO.sub.2 contents for high negative anomalous partial dispersions to be achieved. Furthermore, Example Nos. 10 and 11 do not have the ratios necessary for high negative anomalous partial dispersion between the structure-determining components Al.sub.2 O.sub.3 /B.sub.2 O.sub.3, (R.sub.2 O+MO)/B.sub.2 O.sub.3.
The Japanese publication does not reveal any technical teaching according to which it would be possible to obtain glasses having high UV transmission and simultaneously high negative anomalous partial dispersion.
DE-B 1 303 171 relates to a glass batch for the production of optical glasses having anomalous partial dispersion and Abbe numbers .nu..sub.e of from 40 to 60 and refractive indexes n.sub.e of from 1.52 to 1.64, where the batch contains (in % by weight): SiO.sub.2 21.9-40; B.sub.2 O.sub.3 24.0-34.0; Li.sub.2 O 2.0-10.0; Al.sub.2 O.sub.3 3.8-13.0; Ta.sub.2 O.sub.5 2.8-20.8; ZnO up to 5.1; Na.sub.2 O up to 12.0; ZrO.sub.2 up to 7.5; WO.sub.3 up to 15.0.
DE-B 1 022 764 discloses an optical glass of anomalous partial dispersion which has been produced from boric acid and at least one alkali metal oxide and/or an oxide of a divalent element as the basic substance, the glass comprising from 45 to 80 mol per cent of boric acid, 5-18 mol per cent of alkali metal oxide and/or an oxide of a divalent element and 2-45 mol per cent of oxides of lanthanum, tantalum, niobium and/or lead phosphate.
DE-B 1 303 171 and DE-B 1 022 764 differ from the present invention through essential glass components and their ratios to one another. They likewise fail to achieve the desired magnitude of the negative anomalous partial dispersion.
DE 4 032 567 A1 discloses a glass having negative anomalous partial dispersion in the blue region, a refractive index n.sub.d of 1.69-1.83 and an Abbe number .nu..sub.d of 29-38.5, which contains, in % by weight based on oxide, 0-7.5 SiO.sub.2 +GeO.sub.2, 25-33 B.sub.2 O.sub.3, 19-50 PbO, 0-2 HfO, 1-6 ZrO.sub.2, 1-6 ZrO.sub.2 +HfO.sub.2, 4-20 Ta.sub.2 O.sub.5, 0-8.0 Al.sub.2 O.sub.3, 0-4 TiO.sub.2, 0-1 WO.sub.3, 0-7 Nb.sub.2 O.sub.5, 0-2 GeO.sub.2, 0-4.5Li.sub.2 O, 0-4.5 Na.sub.2 O, 0-4.5 K.sub.2 O, 0-4.5 .SIGMA. alkali metal oxides, 0-20 .SIGMA. alkaline earth metal oxides +ZnO, 0-&lt;9 La.sub.2 O.sub.3, 0-5.5 Y.sub.2 O.sub.3, 0- 5.5 Gd.sub.2 O.sub.3, 0-11 .SIGMA. La.sub.2 O.sub.3 +Y.sub.2 O.sub.3 +Gd.sub.2 O.sub.3,0-10 MgO, 0-16 CaO, 0-16 BaO, 0-16 SrO, 0-16 BaO+SrO, 0-16 ZnO.
This glass is a flint glass having a very high PbO content.